Now, Fitbit is facing a class action lawsuit alleging that its heart rate monitoring technology— which it calls PurePulse—is inaccurate, as first reported by The Verge. Essentially, Fitbit uses LED lights to monitor blood flow through a user’s wrist and, by using algorithms, can determine a user’s heart rate. Similar technology is used in the Apple Watch, along with several more competing smartwatches and fitness trackers.

According to the suit, three different plaintiffs said they bought Fitbit devices based off the company’s advertised promise that it accurately tracked heart rates. Fitbit’s advertising materials often contain phrases such as “every beat counts” and “know your heart” in relation the heart monitoring technology.

In the suit, consumers from California, Colorado, and Wisconsin allege that the heart rate tracking in the Charge HR and the Surge, two products that came to market last year, is inaccurate by a “significant margin,” especially during periods of intense exercise. The plaintiffs say that despite Fitbit’s high-visibility advertising campaign around the heart rate tracking in its products, the monitors don’t actually “count every beat.”

One plaintiff said she bought a Charge HR because she wanted to track her heart rate accurately during exercise, but in her experience the heart rate monitors in the device didn’t work properly. When she complained, Fitbit refused to refund her, she said.

It’s not the first time Fitbit has faced a class-action suit. In the spring of 2014, consumers filed a suit alleging that the company misled them in the advertising of the Fitbit Force wristband after a number of wearers suffered skin rashes from the device. The Fitbit Force was eventually pulled from the market, and Fitbit later re-introduced a similar wristband called the Charge, which the company said had new materials that had undergone enhanced testing.

IT’S NOT THE FIRST CLASS-ACTION SUIT FOR FITBIT

The new lawsuit seeks injunctive relief as well as actual damages payments to compensate users for their “economic injuries” from Fitbit’s heart-rate claims.

Fitbit said in a statement that it does not believe the case has merit and that it stands by its heart rate technology. It “strongly disagrees with the statements made in the complaint and plans to vigorously defend the lawsuit…PurePulse [Fitbit’s proprietary heart rate tech] provides better overall heart rate tracking than cardio machines at the gym, as it tracks your heart rate continuously even while you’re not at the gym or working out. But it’s also important to note that Fitbit trackers are designed to provide meaningful data to our users to help them reach their health and fitness goals, and are not intended to be scientific or medical devices.”

It’s been a rocky week for Fitbit. Its newest watch, the Fitbit Blaze, was met with a lukewarm response after its unveiling yesterday, and Fitbit’s stock dropped more than 18 percent, the lowest it has been since the company went public last summer.

This is partly due to the fact that many people see the new watch as directly competitive with Apple Watch. Like the Apple Watch, the Fitbit Blaze has a watch form factor, a color display, and the ability to track several different workout activities. However, Fitbit is not categorizing the Blaze as a “smartwatch” like the Apple Watch, because the Blaze doesn’t support third-party apps, only Fitbit’s own apps. Fitbit also works with different mobile operating systems, whereas Apple Watch works with iPhones.